In addition to the sharing of intravenous injection equipment, a drug-abuse vector for HIV transmission involves the association of psychomotor stimulant abuse with increased HIV sexual risk behavior and HIV infection. Little is known about decision-making processes contributing to sexual risk behavior, particularly in stimulant abuse. A decision-making process of potential relevance to sexual risk behavior is delay discounting, which is considered an aspect of impulsivity. Delay discounting refers to the observation that delaying a reward reduces its subjective value, and is typically indexed in laboratory studies by preference for smaller sooner over larger later rewards. Delay discounting has been shown to be widely relevant to drug abuse, primarily by studies showing that drug-abusing individuals have higher discounting rates (i.e., are more impulsive, or value future consequences less) than non-abusing individuals. Several animal studies show that chronic exposure to cocaine causes long lasting increases in delay discounting for food reinforcers (i.e. preference for smaller sooner over larger later food), even after prolonged abstinence from cocaine. The Principal Investigator has developed a novel task assessing the discounting of delayed sexual rewards. The task uses clinically relevant hypothetical choices between unprotected sex now vs. waiting for sex with a condom, in reference to the photograph of an individual judged to be sexually desirable by the participant. Preliminary data in stimulant-abusing individuals are systematic, showing that the discounting of sexual rewards conforms to the same mathematical function characteristic of the discounting of other reinforcers in humans and animals. Furthermore, discounting of sexual rewards shows good test-retest reliability at a 1-week interval, and shows sensitivity to two manipulations. Participants were more likely to prefer immediate unprotected sex in response to more desirable partners, and in response to partners judged least likely to have a sexually transmitted disease. This study will systematically extend this line of research by examining the effects of two doses of oral methamphetamine and placebo in stimulant-abusing participants in the laboratory while assessing decision making for delayed hypothetical sexual and real money rewards. A measure of subjective sexual arousal will provide data on whether any drug effects on discounting are related to or independent of increased sexual motivation resulting from the drug. The study will provide the first experimental evidence of potential effects of methamphetamine that might contribute to sexual risk behavior (increased delay discounting for sexual rewards, increased delay discounting for rewards generally, increased sexual arousal). Ultimately, completion of this project will increase our understanding of sexual HIV risk behavior, inform HIV prevention and education efforts, and provide a new and empirically developed tool for examining sexual risk behavior.